In modern semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of such devices. Conventionally, a flip-chip attachment method has been used in the packaging of IC chips. In the flip-chip attachment method, instead of attaching an IC die to a lead frame in a package, an array of solder balls is formed on the surface of the die. The formation of the solder balls is normally carried out by through-mask evaporation, solder paste screening, or injection molding of solder.
U.S. Pat. No. 5,244,143, which is commonly owned by International Business Machines Corporation, discloses the injection molded solder (IMS) technique and is hereby incorporated by reference in its entirety. One of the advantages of the IMS over other solder bumping techniques is that there is very little volume change between the molten solder and the resulting solder bump. The IMS technique utilizes a solder head that fills boro-silicate glass (or other material) molds that are wide enough to cover most single chip modules. A wiper is sometimes provided behind the solder slit passes the filled holes of the mold to remove excess solder.
The IMS method for solder bonding is then carried out by applying a molten solder to a substrate in a transfer process. When smaller substrates, i.e., chip scale or single chip modules are encountered, the transfer step is readily accomplished since the solder-filled mold and substrate are relatively small in area and thus can be easily aligned and joined in a number of configurations. For instance, the process of split-optic alignment is frequently used in joining chips to substrates. The same process may also be used to join a chip-scale IMS mold to a substrate (chip) which will be bumped. One problem with current IMS systems are the fill heads uses to place solder in the cavities of the molds. These fill heads are restricted to linear deposition of solder into rectangular molds. That is, the mold and the solder head are moved linearly with respect to each other such that the cavities move perpendicular to a slit in the solder head thereby filling the cavities as they pass. Another problem with IMS is that the molds are limited to a rectangular configuration, which encourages the linear deposition of the solder.
Another problem with current fill heads used for IMS and other solder bumping techniques is that they do not provide precise temperature control with response time required to accurately control solder melt and solidification. Current fill heads are designed with resistive (electric) heaters in the solder head. The heater is built into the surface of the fill head where the head contacts the substrate being filled. The performance of a heater of this design is limited by a time delay for the heat to be generated in the line and then the conduction of the fill head. Also, there is a time delay for cooling of the solder in the cavities because the fill head does not provide a means for cooling the solder.
Therefore a need exists to overcome the problems with the prior art as discussed above.